U.S. patent number 3,858,977 [Application Number 05/400,939] was granted by the patent office on 1975-01-07 for optical interference authenticating means.
This patent grant is currently assigned to Canadian Patents and Development Limited. Invention is credited to Kenneth M. Baird, Philip D. Carman, Jerzy A. Dobrowolski, Allan J. Waldorf.
United States Patent |
3,858,977 |
Baird , et al. |
January 7, 1975 |
OPTICAL INTERFERENCE AUTHENTICATING MEANS
Abstract
An optical interference, authenticating means, comprising a
substrate, and a filter composed of at least one optical
interference layer having a known characteristic of spectral
reflectance and a different, known characteristic of spectral
transmittance, both of which vary with the angle of incidence of
light on the filter. The substrate, which may be a portion of a
valuable document such as a bank note, has a portion of the surface
adjacent to the filter coloured to absorb some of the light
transmitted by the filter.
Inventors: |
Baird; Kenneth M. (Ottawa,
CA), Dobrowolski; Jerzy A. (Ottawa, CA),
Waldorf; Allan J. (Kemptville, CA), Carman; Philip
D. (Ottawa, Ontario, CA) |
Assignee: |
Canadian Patents and Development
Limited (Ottawa, CA)
|
Family
ID: |
27161703 |
Appl.
No.: |
05/400,939 |
Filed: |
September 26, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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305069 |
Nov 9, 1972 |
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Foreign Application Priority Data
Current U.S.
Class: |
356/71; 359/582;
359/588 |
Current CPC
Class: |
G07D
7/1205 (20170501); B42D 25/29 (20141001); G02B
5/285 (20130101) |
Current International
Class: |
G02B
5/28 (20060101); G07D 7/00 (20060101); G07D
7/12 (20060101); G01k 009/08 () |
Field of
Search: |
;356/71
;350/164,166,318 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: McGraw; Vincent P.
Attorney, Agent or Firm: Lemon; Francis W.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
305,069, now abandoned.
Claims
We claim:
1. An optical interference authenticating means, comprising a
substrate integral with an article to be authenticated, and a
filter composed of at least one optical interference layer
overlying and attached to the said substrate, said filter having a
known characteristic of spectral reflectance, and a different,
known characteristic of spectral transmittance both of said
characteristics varying with the angle of incidence of light on the
said filter, and said substrate having at least a portion of the
surface thereof, adjacent to said filter, coloured a particular
colour to absorb at least some of the light transmitted through the
said filter.
2. A means according to claim 1, wherein said surface adjacent to
said filter is rough for diffuse reflection therefrom of light
transmitted by said filter and not absorbed by said surface, and
said filter is attached to the said substrate by a transparent
cement.
3. A means according to claim 1, wherein said surface adjacent to
said filter is substantially smooth for specular reflection.
4. A means according to claim 1, wherein the/or each optical
interference layer is of a material selected from the group
consisting of cryolite, MgF.sub.2, SiO, SiO.sub.2, ThF.sub.4, ZnS,
TiO.sub.2, ZrO.sub.2, Ag, Al, Au, Cu, Cr, Ge, Ni, NiCr and Si.
5. A means according to claim 1, wherein at least a portion of the
surface of said substrate, adjacent said filter, is coloured
black.
6. A means according to claim 1, wherein an outer, transparent or
coloured protective film is provided on the said filter.
7. A means according to claim 1, wherein said substrate is an
adhesive layer on said article to be authenticated and attaching
the said filter thereto.
8. A means according to claim 1, wherein said substrate is a film
carrier layer integral with said article to be authenticated.
Description
This invention relates to an optical interference, authenticating
means.
Counterfeiting of valuable papers and of identification documents
made of paper or plastics, such as bank notes, credit cards,
passports and passes is presently inhibited more or less
successfully through the use of inks of many colours and intricate
engraved designs on special papers which sometimes contain
watermarks or embedded coloured paper platelets or metal threads.
These methods have not always offered sufficiently discriminating
tests to prevent, in some instances, spurious bank notes and other
documents from being passed to the man in the street, in shops, and
even in banks.
It is one object of the present invention to provide an
authenticating means, which may incorporate or be used in
conjunction with one or more of the above mentioned methods to
provide better protection against the performance of counterfeiters
and forgers.
Another object of the present invention is to provide an
authenticating means which provides more discriminating tests of
the authenticity of a valuable paper or identification document for
the man in the street, for the shop or bank, and for the crime
laboratory than known types of authenticating means.
It would be desirable to provide an authenticating means for, say,
valuable papers and identification documents whereby the
authenticating equipment makes use of new technologies and requires
large capital outlays that are beyond the means of most
counterfeiters.
A further object of the present invention is to provide an
authenticating means for, say, valuable papers and identification
documents which makes use of a relatively new technology for the
manufacture of the authenticating means, requiring large capital
outlays for equipment that are beyond the means of most
counterfeiters.
The present invention, in some embodiments, makes use of particular
optical thin film multilayer coatings hereinafter referred to as
optical interference layers. In recent years advances have been
made in the preparation of optical thin film coatings by methods
such as vacuum evaporation, sputtering, chemical deposition from
vapours or organic solutions, see, for example, L. I. Maissel and
R. Glang, "Handbook of Thin Film Technology," McGraw-Hill, 1970.
Consequently such coatings find increasing use as optical filters.
By depositing onto a suitable substrate a number of layers made of
suitable materials and having appropriate thicknesses it is
possible to make antireflection coatings, high reflectance
coatings, beam splitters, heat reflectors, narrow band pass
filters, etc., see, H. A. MacLeod, "Thin Film Optical Filters,"
American Elsevier, New York 1969.
Spectral reflectance and spectral transmission characteristics
depend on the optical constants, refractive indices, absorption
coefficients and thicknesses of the optical thin film coatings, but
as is known to persons skilled in the art, these characteristics
are affected by other factors, for example, angle of incidence and
state of polarization of the incident light, and materials (in film
or bulk form) surrounding the layers, and so these other factors
must also be taken into account when observing these
characteristics.
One property of such multilayer optical filters is that their
spectral reflectance and transmittance characteristics vary with
the angle of incidence of the light on the coating. This means that
the observed hues of the light transmitted or reflected by the
filter will also vary. For all the above mentioned applications
this variation in hues is a decided disadvantage and efforts are
often made to minimize these effects.
The authenticating means according to the present invention also
makes use of optical coatings or multilayer coatings. However,
unlike in the case of the filters described above, the observed
variation in the colour with angle of incidence of the light
transmitted or reflected by the optical interference layers of
authenticating means according to the present invention is
desirable and efforts should be made to maximize it. For in this
respect the authenticating means according to the present invention
differ essentially from all known types of dyes, inks, pigments and
paints used in printing, photography and colour copying
machines.
Thus in this specification "optical interference layers" are
defined as optical thin film multilayer coatings wherein the
observed variation in the hue with angle of incidence of the light
transmitted or reflected by the optical thin film multilayer
coatings is significant.
According to the present invention there is provided an optical
interference, authenticating means, comprising a substrate, and a
filter composed of at least one optical interference layer
overlying and attached to the said substrate, said filter having a
known characteristic of spectral reflectance and a different known
characteristic of spectral transmittance, both of said
characteristics varying with the angle of incidence of light on the
said filter, and said substrate having at least a portion of the
surface thereof, adjacent to said filter, coloured a particular
colour to absorb some of the light, which is transmitted through
the said filter.
By using the conventional thin film design techniques reviewed by
H. A. MacLeod referred to above or an automatic thin film synthesis
program described by J. A. Dobrowolski in Applied Optics 4,937
(1965); ibid 9, 1936 (1970) the thicknesses and refractive indexes
of the optical interference layers may be chosen to yield a strong
coloration of the transmitted and reflected beam that varies
rapidly in hue with change in the angle of incidence of the
beam.
The present invention makes use of the fact that the reflected
colour from a white light that one would actually perceive when
such a filter is attached to an opaque substrate depends strongly
on the colour of the surface of the substrate. For example, a free
standing nine layer filter composed of alternate optical
interference layers of zinc sulphide and magnesium fluoride may
have a high reflectance in the red part of the spectrum, and when
viewed at increasing angles of incidence the wavelength of the main
reflectance peak shifts from the red through the yellow and green
towards the blue part of the spectrum. But the colour that one
would actually perceive when such a coating is attached to an
opaque surface will depend strongly on the spectral absorption
characteristics and thus colour of that surface. Since at normal
incidence the filter will strongly reflect red light, light of
shorter wavelengths will be transmitted by the filter and will fall
on to the opaque surface. Some of the light falling on the opaque
surface is reflected back by this surface and transmitted out again
by the filter, and if this is substantially all of the light
transmitted by the filter, then the reflected light from the
surface will recombine with the reflected light from the filter to
produce, what is observable by the eye as, white light once
more.
If a suitable combination of optical interference coatings and
surface colour and pattern of the substrate are chosen, it is
possible to obtain interesting colour effects. A maximum colour
contrast is obtained where the surface of the substrate is coloured
black. On the other hand, no colour is observed where the substrate
is coloured white because the white colour substantially does not
absorb. Thus, a black silhouette on a white background on the
substrate will only show the silhouette brightly revealed in the
hue reflected by the filter.
As this effect cannot be duplicated by any paint, pigment or colour
copying procedure it should therefore enable the man in the street
to distinguish at a glance an authentic valuable paper bearing such
optical interference layers from even the best counterfeit lacking
this authenticating device.
In the accompanying drawings which illustrate, by way of example,
the embodiments of the present invention,
FIG. 1 is a front view of a Canadian Twenty dollar bank note,
bearing optical interference, authenticating means,
FIGS. 2a-2c are enlarged, schematic cross-sections of different
embodiments of a multi-layer optical interference, authenticating
means deposited onto a suitable substrate, as might be applied to
the bank note of FIG. 1,
FIG. 3 is a CIE chromaticity diagram,
FIGS. 4 and 5 represent the spectral reflectance curves for three
different angles of incidence of light of two optical interference,
authenticating means suitable for use in reflected light,
FIGS. 6 and 7 illustrate the action of two special optical
interference, authenticating multilayer coatings which permit a
positive indentification not dependent on colour vision, and
FIG. 8 represents the spectral reflectance characteristics of an
optical interference, authenticating means designed to be difficult
to imitate even by counterfeiting operations with fairly large
financial resources.
FIG. 1 represents an opaque Canadian twenty dollar bill 1 with
optical interference, authenticating means each in the form of
coatings 2 and 3 applied to the front side, and similar coatings 4
and 5 (shown dotted) applied to the rear side of the bank note. If
the authenticating coatings are arranged in this way, then in
whatsoever a manner a number of bank notes 1 are assembled as a wad
a coating 2, 3, 4, or 5 will always appear in the top left hand
corner, and another one will always appear in the lower righthand
corner of the wad. Thus, without arranging all of the bank notes 1
of a wad face upwardly, or with all of them with the figures up the
right way, it is possible to flick through the corners to reveal a
coating 2, 3, 4 or 5 and to detect at a glance spurious bank
notes.
Referring now to FIG. 2a, each optical interference, authenticating
means or coating 2 to 5 (FIG. 1) consists of a substrate 13 and a
plurality of optical interference layers 6 to 12 which form a
filter 6a. The filter 6a has a known characteristic of spectral
reflectance, and a different, known characteristic of spectral
transmittance. The substrate 13 has at least a portion of its
surface 14 coloured a particular colour to absorb at least some of
the light transmitted through the filter 6a. Each layer 6 to 12 may
be deposited by one of the techniques mentioned above, directly
onto the substrate, which is part of the document to be protected,
if the surface finish of the latter is good enough. But it is often
more convenient and satisfactory to first deposit the layers onto a
suitable carrier 26 (FIG. 2b) in the form of a transparent or
coloured polyester film. When the filter 6a is deposited on a
polyester film it may be manufactured in one length which is then
cut into small strips to form the coatings 2 to 5. The coatings 2
to 5 may then be secured to the valuable paper by means of a
suitable adhesive 25 which must be transparent if the colour of the
substrate surface 14 is to interact with the light transmitted by
the filter 6a. In another embodiment of the invention the adhesive
25 may contain a dye and then the adhesive itself will provide the
coloured surface 14 necessary for obtaining colour contrast. The
adhesive 25 is applied to the layer 12 and the polyester film 26
which is in contact with layer 6, may be retained to provide
protection to the filter 6a as a protective covering.
Alternatively, the polyester carrier 26 can be removed after the
filter 6a has been attached to the substrate (FIG. 2c).
In another embodiment the carrier 26 of FIG. 2b forms the substrate
13 shown in FIG. 2c the carrier 26 is then for attachment to an
article to be authenticated.
With the filter 6a disposed in the path of a light beam, with the
light of the light beam impinging on all of the layers 6 to 12, the
filter 6a will reflect light of a certain hue and transmit light of
a different hue and this will vary with the angle of incidence of
light thereon. However, the surface 14 is coloured a particular
colour to absorb some of the light transmitted through the filter
6a.
The sensitivity of the human eye to light of different colours
varies greatly. Quite conceivably a fairly low secondary
reflectance maximum in the green part of the spectrum might have a
greater affect on the colour that is perceived by the human eye
than the main red maximum. An appropriate way of describing the
behaviour of the filter 6a is with the aid of the CIE chromaticity
diagram shown in FIG. 3. The curved boundary 15 is the spectrum
locus, that is, the position on the diagram of pure colours. All
real colours lie within the area bounded by the curved boundary 15
and the line 16 joining the blue and red ends of the curved
boundary 15. The white patch 17 represents the achromatic point.
All points on a line joining the achromatic point or white patch 17
and a point on the spectrum locus or curved boundary 15 have the
same hue or dominant wavelength and differ from one another only by
the degree of saturation or purity. Pale and impure colours lie
close to the achromatic point or white patch 17, purer colours lie
close to the spectrum locus or curved boundary 15.
Two samples may be the same colour, yet one may appear very bright
and the other almost black. To indicate this quantitatively the
luminous reflectance can be specified.
Besides the colour of the surface of the substrate, another
circumstance that will affect the appearance of the
coating/substrate combination is whether the substrate reflects
specularly or diffusely. Diffuse reflection takes place when the
coating is attached with a transparent cement to a rough surface.
Approximately specular reflectance is observed when the surface 14
is substantially smooth, and this may be achieved when the adhesive
used to attach the coating to the substrate contains a dye or
pigment.
To summarise, the visual appearance of the filter/substrate
combination can be specified by a point on the CIE chromaticity
diagram shown in FIG. 3, and this method has the advantage that is
reveals a minimum of information about the structure of the
filter.
The layers 6 to 12 (FIG. 2) may be made of any of the commonly used
non-absorbing optical coating materials such as, for example,
cryolite, MgF.sub.2, SiO, SiO.sub.2, ThF.sub.4, TiO.sub.2, ZnS and
ZrO.sub.2, or absorbing materials such as, for example, Ag, Al, Au,
Cu, Cr, Ge, Ni, NiCr, and Si, or any other materials that form
satisfactory layers or coatings. The thicknesses of the layers will
normally be within one or two orders of magnitude of 0.1.mu.m.
As explained above, the spectral reflectance and transmittance of
an optical thin film system depends on the number of layers, their
thicknesses and the materials used for their construction. By a
suitable choice of these parameters varous different effects can be
achieved. By way of example, the construction parameters of the
layers of a filter of an authenticating means are shown in Tables
I, and II respectively. In both cases the aim was an authenticating
means that would appear red when viewed normally and whose peak
reflectance would, on tilting, gradually shift towards the blue
part of the spectrum. For maximum colour effect the width of the
spectral region isolated should not be too wide and the transition
from low to high reflection should be sharp. The system of Table I
utilizes only materials for the layers that do not absorb in the
visible part of the spectrum. Such a system is usually more
efficient than that given in Table II which also uses absorbing
materials. However, the overall thicknesses of systems containing
absorbing layers are usually considerably thinner, and consequently
such coatings should be cheaper to produce.
TABLE I ______________________________________ Construction
parameters of the filter of FIG. 4. Layer Thickness Material number
(in .mu.m) ______________________________________ 1 0.204 ZnS 2
0.355 MgF.sub.2 3 0.204 ZnS 4 0.355 MgF.sub.2 5 0.204 ZnS 6 0.355
MgF.sub.2 7 0.204 ZnS 8 0.355 MgF.sub.2 9 0.204 ZnS
______________________________________
TABLE II ______________________________________ Construction
parameters of the filter of FIG. 5. Layer Thickness Material number
(in .mu.m) ______________________________________ 1 0.216 Al 2
0.010 Ni 3 0.220 SiO.sub.2 4 0.005 Ni 5 0.250 SiO.sub.2 6 0.008 Cu
______________________________________
In FIG. 4 are shown the calculated reflectance curves 18, 19, 20
for 0.degree., 30.degree., and 45.degree. incidence for an
authenticating means of the type shown in Table I. It should be
remembered, however, that because only non-absorbing layer
materials were used in the construction of the optical interference
layers of the filter, the viewer will see, in addition to the light
reflected by these optical interference layers, light reflected by
a surface layer of the substrate. As stated above, interesting
colour effects can be obtained with a suitable combination of
colours of substrate, which may be a portion of a document, and of
the characteristics of the optical interference layers. However, if
maximum colour contrast is desired, the surface layer of the
portion of the surface of the substrate to reflect light should
preferably be black.
The authenticating means whose constructions parameters are shown
in Table II utilizes light absorbing layer materials as well as
non-light-absorbing materials in such a way as to obtain a nearly
square shaped reflection band. In this it differs from the narrow
band reflection filters such as those described by A. F. Turner and
H. R. Hopkinson, J. Opt. Soc. Amer. 43, 819 (1953). The
authenticating means is opaque and so it does not require a black
background for maximum contrast. In FIG. 5 are shown the calculated
reflectance curves 21, 22, 23 for an authenticating means of this
type for light incident at 0.degree., 30.degree., and 45.degree.
respectively.
Optical interference, authenticating means of the type illustrated
in Tables I, and II, and in FIGS. 4 and 5 are to be construed only
to be examples of the very many different optical interference,
authenticating means according to the present invention that can be
made. Thus, for instance, it is a simple matter to design layers or
coatings that are peaked at different parts of the spectrum and
that on tilting undergo a different sequence of colour changes.
The principle of special authenticating means, according to the
present invention, whose operation is not dependent on colour
vision is illustrated by the spectral reflectance curves 29, 30
shown in FIG. 6. The authentication means comprises a filter which
has very little reflectance in the visible part of the spectrum but
has a high reflectance in the near infrared spectral region, on a
substrate having a black coloured surface adjacent the filter. The
transition from the low to the high reflectance regions should be
sharp for maximum effect. The black colour may be printed on a
portion of the valuable paper or identification document forming
the substrate and little light will be reflected at normal
incidence. On tilting the high reflection region will move into the
visible part of the spectrum and a strong reflection will be
observed.
The principle of another special authenticating means, according to
the invention, whose operation is not dependent on colour vision is
illustrated by the spectral transmittance curves 31, 32 shown in
FIG. 7. This authenticating means comprises a filter in the form of
a broadband high reflectance coating for the visible part of the
spectrum with a sharp transition to a high transmittance in the
near infrared region and a substrate having the surface adjacent
the filter coloured with patterns or inscriptions. At normal
incidence (curve 31) the coating would look like a mirror. On
tilting (curve 32) the transmission region would gradually shift
into the visible spectral region and patterns and inscriptions of
the substrate which may be a printed portion of the valuable paper
or identification document would become visible.
Curve 33 in FIG. 8 illustrates the fact that it is possible to
construct filters of multilayers or coatings with quite complicated
spectral transmittance or reflectance characteristics. Designs with
certain predefined spectral reflectance curves can be obtained
through the use of automatic thin film synthesis programs. The
present day state of the art of spectroscopic analysis and electron
microscopy seems inadequate to deduce completely the design of such
complicated multilayers or coatings. Accordingly optical
interference, authenticating means of this type would be difficult
to imitate even by counterfeiting operations backed by unlimited
financial resources. Using an automatic thin film synthesis program
it might be possible to generate an optical interference,
authenticating means whose spectral characteristics were a rough
approximation in a certain spectral region of those of the
original. However by examining it over a more extended spectral
region and for different angles of incidence of the light using
routine laboratory spectrophotometric techniques, one would detect
differences. To arrive at the same design of optical interference,
authenticating means it would be necessary not only to use the same
automatic thin film synthesis program that was used to generate the
prototype, but also to apply it to the problem in exactly the same
way.
In another possible application a filter could be applied over a
substrate bearing a signature or photograph on a valuable paper or
identification document to afford protection against the forging of
the signature or the substitution of photographs. For this
application a combination of adhesives and coating materials
forming the filter would be chosen such that the filter part of the
optical interference, authenticating means could not be removed or
transferred without being destroyed or damaged.
* * * * *